Chemical Clock

 Let us view the concept of a chemical clock

 no...not that type of chemical clock.

I am talking about a scenario where chemical compounds react and bring with them sudden, observable, events that can be tracked quiet precisely over a given amount of time. For example, think of a solution going from clear to black, and then back to clear again. Such events can be tracked at specific time intervals. 

Ahh yes. The good old iodine clock. (a.k.a the Harcourt-Esson Reactions) 

This reaction is started by mixing two solutions together. The first is a solution of hydrogen peroxide and sulfuric acid. To this solution we add another consisting of potassium iodide, sodium thiosulfate, and good old starch.

Once these lovelies are mixed we get these wonderful reactions:

In the first, slow reaction, the triiodite(I love this word) ion is produced:

H₂O₂ + 3 I⁻ + 2 H⁺ → I₃⁻ + 2 H₂O

In the second, fast reaction, triiodide is reconverted to iodide by the thiosulfate:

I₃⁻ + 2 S₂O₃²⁻ → 3 I⁻ + S₄O₆²⁻

But, alas, chemistry is all about our states of equilibrium. The thing is that the triiodide ion starts to be consumed faster than it can be generated when the solutions are mixed.  So, this second reaction is responsible for our dark blue/black color that seems to happen in a blink of an eye.

And then we find the equilibrium problem again and resort back to our clear color in different variations of this experiment. For in some reactions, the solution cycles from dark blue to clear until the reagents are all depleted.